1. Field of the Invention
The present invention relates to a recording medium for ink, particularly a recording medium for ink suitable for ink jet recording, and a method for producing a recording medium for ink. More specifically, the present invention relates to a novel recording medium which is improved with respect to the displacement of a coloring agent in the recording medium, generated in an image after the coloring agent contained in an ink is received as a fixed image, and a substrate (base paper) on which an ink receiving layer is to be formed. The present invention provides a recording medium for ink capable of suppressing image deterioration resulting from coloring agent (dye) displacement, which tends to be caused particularly during storage in a high humidity environment, or image deterioration caused by light when a printed image is displayed, thereby providing a printed image exhibiting excellent stability over time. The present invention also relates to a method for producing a recording medium for ink.
2. Related Background Art
Recently, as a recording medium for ink having a high ink absorbability and capable of providing a high quality image, there is being commercialized a recording medium for ink having an ink receiving layer which employs an alumina hydrate as an inorganic pigment. The recording medium for ink having the ink receiving layer employing such alumina hydrate, because of a positive charge of the alumina hydrate, shows a satisfactory fixation of the dye constituting the coloring agent in the ink and a satisfactory transparency, thereby providing advantages in comparison with the prior art recording media for ink such as a higher print density, an improved color development in the image, a higher surface glossiness and an image of photographic gradation.
Also, because of the increasing popularity of digital cameras, the recording medium for ink for recording the image information thereof is required to have a photograph-like glossy feeling in addition to a high image quality, in order to obtain an image close to a silver halide photograph. In the recording medium for ink having an ink receiving layer employing an alumina hydrate as mentioned above, if the substrate is a film, the ink receiving layer is formed by applying a coating liquid containing the alumina hydrate to obtain an image having a photographic glossiness. On the other hand, if the substrate is a fiber-based one, namely paper, the photographic glossiness cannot be obtained even by employing a coating liquid containing alumina hydrate for forming the ink receiving layer, and a super calendering or a cast coating is employed for providing the ink receiving layer with a gloss. In such case, a glossy image closer to the silver halide photograph can be obtained by a recording medium for ink processed by the cast process.
Such ink receiving layer containing alumina hydrate has a coloring agent fixing function because alumina functions as a cationic reactive material for an anionic dye, and, in order to further improve such function, Japanese Patent Application Laid-open Nos. 9-66663 (reference 1) and 2001-341412 (reference 2) disclose inventions for enhancing such function. Reference 1 describes application of a cationic treatment also on an outermost layer, and reference 2 describes application of a cationic treatment on the fibers of the substrate. Also, Japanese Patent Application Laid-open No. 8-230311 (reference 3) discloses a technology of causing a cationic surfactant to penetrate into an outermost layer.
On the other hand, to counteract the so-called rear penetration phenomenon caused by penetration of the recorded ink through the substrate and a re-displacement of the coloring agent, reaching the substrate, by deposition of water, Japanese Patent No. 3204749 (reference 4) discloses an invention of providing a layer containing a cationic polymer resin between the substrate and the ink receiving layer. According to this invention, after coating and drying of a coating liquid in which a water-soluble cationic polymer resin is dissolved in water, an ink receiving layer is formed with a coating liquid containing a pigment such as silica and a binder. This invention provides an intermediate layer containing a cationic polymer in order to intercept the coloring agent that can reach the substrate. Similarly, Japanese Patent Application Laid-open No. 11-105414 (reference 5) describes a recording medium having an undercoat layer between the substrate and the ink receiving layer, in which the undercoat layer contains at least one water-soluble cationic resin.
Also, the recording material for recording the aforementioned image information is being required to have a photograph-like glossiness in order to obtain an image close to a silver halide photograph. In order to meet such requirement, it is already known that a recording medium of a high glossiness can be obtained by employing a cast process on a recording medium having an ink receiving layer employing alumina hydrate and polyvinyl alcohol as binders. In particular, Japanese Patent Application Laid-open No. 2001-138628 (reference 6), for providing a high gloss medium for ink jet recording, discloses an invention of employing re-swelling as a technology for improving the cast process.
Formation of an ink receiving layer employing alumina hydrate and polyvinyl alcohol as binders is already known, but a viscosity increase with time in the coating liquid containing alumina hydrate and polyvinyl alcohol is an important factor. This is partly recognized in Japanese Patent Application Laid-open No. 7-76161 (reference 7). The reference 2 proposes, in order to solve the problem of fine cracks generated in drying the above-mentioned coating liquid, an alumina sol coating liquid containing alumina hydrate, polyvinyl alcohol and a predetermined amount of boric acid or a borate salt and a resinous film coated with such liquid (however, this reference only considers the coating liquid directly coated on the resinous film and only discloses coating of the liquid in one point where the ink receiving layer is 23 g/m2). On the other hand, Japanese Patent Application Laid-open No. 11-291621 (reference 8) cites the reference 2 and points out the difficulty of stable coating with the coating liquid disclosed in the reference 2 (references 7, 8 being by the same applicant). Reference 8 is based on a technical concept denying the improvement in the coating liquid and discloses a substrate paper which is obtained by sizing and drying a base paper principally formed by paper and prior to the coating process. More specifically, this invention prepares in advance a base paper by drying boric acid or the like of 0.5 to 1.5 g/m2 and a paper surface treating agent (surface reinforcing agent or surface sizing agent) with a size pressing. In an example in reference 8, after the preparation of such base paper, a coating liquid constituted of boemite and polyvinyl alcohol and not containing a crosslinking agent is prepared, and the prepared coating liquid is applied on the aforementioned base paper. The size pressing means, which is generally for improving water resistance, surface smoothness, printability etc. of the base paper, is used to slightly coat or impregnate the surface of the base paper with the sizing agent, which then is dried, for example, with a drum dryer. Therefore, the aforementioned references 7, 8 recognize the viscosity increase in the coating liquid, which is a drawback in the prior technology, but reference 7 merely tries to find a solution in the composition of the coating liquid, while reference 8 merely tries to find a solution in the base paper.